Consolidated Statements of Changes in Equity - USD ($) $ in Thousands	Total	Treasury Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings/(Accumulated deficit)	Common Stock Class A	Preferred Stock	Convertible Preferred Stock Retained Earnings/(Accumulated deficit)
Common stock, shares outstanding, start at Dec. 31, 2015						26,401,163.000
Balance, start at Dec. 31, 2015	$ 789,993	$ (137,356)	$ 903,429	$ 0	$ 23,627	$ 292	$ 1
Preferred stock, shares outstanding, start at Dec. 31, 2015							120,750
Redeemable Non-controlling Interest, start at Dec. 31, 2015	186,911
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) attributable to Cowen Inc.	(19,277)				(19,277)
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and investment funds	6,882
Foreign currency translation	(2)			(2)
Capital contributions	276,923
Capital withdrawals	(18,469)
Deconsolidation of investment funds	(73,042)
Common stock issued during period, value, acquisitions	0
Restricted stock awards issued, shares						1,511,995
Purchase of treasury stock, at cost, shares						(1,181,869)
Purchase of treasury stock, at cost	(16,489)	(16,489)
Preferred stock dividends	(6,792)							$ (6,792)
Embedded Cash Conversion Option	0
Amortization of share based compensation	26,039		26,039
Common stock, shares outstanding, end at Dec. 31, 2016						26,731,289
Balance, end at Dec. 31, 2016	772,650	(153,845)	928,646	(2)	(2,442)	$ 292	$ 1
Preferred stock, shares outstanding, end at Dec. 31, 2016							120,750
Redeemable Non-controlling Interest, end at Dec. 31, 2016	379,205
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Warrants issued	15,200
Adjustment to Additional Paid in Capital, Income Tax Effect from Share-based Compensation, Net	(822)		(822)
Net income (loss) attributable to Cowen Inc.	(60,882)				(60,882)
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and investment funds	23,791
Foreign currency translation	(6)			(6)
Capital contributions	119,112
Capital withdrawals	(81,504)
Deconsolidation of investment funds	0
Common stock issued during period, shares, acquisitions						3,162,278
Common stock issued during period, value, acquisitions	47,607		47,575			$ 32
Restricted stock awards issued, shares						2,060,927
Purchase of treasury stock, at cost, shares						(2,322,474)
Purchase of treasury stock, at cost	(33,001)	(33,001)
Preferred stock dividends	(6,792)							(6,792)
Embedded Cash Conversion Option	0
Amortization of share based compensation	28,443		28,443
Common stock, shares outstanding, end at Dec. 31, 2017						29,632,020
Balance, end at Dec. 31, 2017	$ 748,019	(186,846)	1,004,664	(8)	(70,116)	$ 324	$ 1
Preferred stock, shares outstanding, end at Dec. 31, 2017	120,750						120,750
Redeemable Non-controlling Interest, end at Dec. 31, 2017	$ 440,604
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative Effect on Retained Earnings, Net of Tax	(559)
Net income (loss) attributable to Cowen Inc.	42,819				42,819
Net income (loss) attributable to redeemable non-controlling interests in consolidated subsidiaries and investment funds	40,398
Foreign currency translation	3			3
Capital contributions	114,644
Capital withdrawals	(278,307)
Deconsolidation of investment funds	(32,559)
Common stock issued during period, value, acquisitions	0
Restricted stock awards issued, shares						2,009,435
Purchase of treasury stock, at cost, shares						(3,203,595)
Purchase of treasury stock, at cost	(47,296)	(47,296)
Preferred stock dividends	(6,792)							$ (6,792)
Embedded Cash Conversion Option	21,195		21,195
Amortization of share based compensation	37,018		37,018
Common stock, shares outstanding, end at Dec. 31, 2018						28,437,860
Balance, end at Dec. 31, 2018	$ 794,407	$ (234,142)	$ 1,062,877	$ (5)	$ (34,648)	$ 324	$ 1
Preferred stock, shares outstanding, end at Dec. 31, 2018	120,750						120,750
Redeemable Non-controlling Interest, end at Dec. 31, 2018	$ 284,780